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An important part of our field research is
carried out in the Aranjuez Experimental Station (AES), located in the vicinity
of Aranjuez, 55 km south of Madrid. With
the support of an Early Career Project Grant from the British Ecological Society, and of some start-up funds I get
from Rey Juan Carlos University(URJC) and the Spanish Ministry of Education and
Science, we started
working in the AES in the fall of 2006, and have been continuously working
there since then. The site has hosted more than a dozen of experiments from our
lab and other colleagues from the Biodiversity and Conservation Area of the Rey Juan Carlos University, and has supported seven PhD Theses
so far, both from the Maestre lab(Santiago Soliveres, Andrea
Castillo-Monroy, Manuel Delgado-Baquerizo, Cristina Escolar, and Miguel Berdugo)
and from other groups that are collaborating with us (Guadalupe León from CEBAS-CSIC
and Mónica Ladrón de Guevara, from EEZA-CSIC). Since 2010, all our research
activities at the AES have been funded by the Starting Grant BIOCOM, awarded to Fernando T. Maestre by the European
Research Council. The
research carried out at the AES has been derived in 15 articles published in
international peer-reviewed scientific journals (see references below), and is
also very important for the teaching and outreaching activities carried out by the
lab. The AES is visited every year by students from a Master in Restoration Ecology, who learn about the research we carry out
there and use also the AES for their MsC. theses, by national and international
visitors coming to the Maestre lab (click here for some pictures), and by members
of local NGOs interested in nature conservation.

The AES is fundamental for us as a research group,
as it is our main research site and a fantastic natural laboratory to study the
ecology of drylands. As a small homage and recognition to the AES and to the
people that has been working there over the last decade, in this blog post we
describe the main characteristics of the AES, describe past/current experiments
and synthesize some of the main results we have obtained from the research
carried out there so far.

Description of the Aranjuez Experimental
Station

The Aranjuez Experimental Station (AES) is
located in the centre of the Iberian Peninsula (40º02’ N – 3º 32’W; 590 m
a.s.l., Figure 1). It is part of the “Finca de Sotomayor”, a larger area historically
devoted to host the Royal stable (“Caballerizas reales”) and in recent times to
small game hunting and to the growth of grain crops typical from semi-arid
Mediterranean areas. This land is owned by the Regional Government (Comunidad
de Madrid), which has managed the site through a research institute devoted to
agriculture, livestock and rural development (“Instituto Madrileño de
Investigación y Desarrollo Rural, Agrario y Alimentario, IMIDRA)”. The AES is part of the European Union Natura 2000 network of protected areas, as it is included within
the Special Protection Areas for Birds “Carrizales y Sotos de Aranjuez” (ES
0000119) and the Site of Community Importance“Vegas, Cuestas y Páramos del Sureste” (ES3110006).

Figure
1.
Location of the Aranjuez Experimental Station and view of the plant and
biological soil crust communities that dominate it. Source: Delgado-Baquerizo
(2013).

The climate of the AES is Mediterranean
semi-arid, with a mean annual temperature and rainfall of 15ºC and 349 mm,
respectively (Aranjuez Meteorological Station, 40º02’ N – 3º 32’W; 540 m a.s.l.;
average data from the 1983-1988 and 1997-2011 periods). The soil is derived
from gypsum, and is classified as Xeric Haplogypsid. Perennial plant cover is
below 40%, and is dominated by the tussock grass Stipa tenacissimaL. (18% of total cover) and the N-fixing shrub Retama sphaerocarpa (L.) Boiss(6% of total cover). Other shrubs
common at the AES are Helianthemum
squamatum (L.) Dum. Cours and Lepidium subulatum L. The open areas between perennial plants are
colonized by well-developed biological soil crusts (BSCs) dominated by lichens
such as Diploschistes diacapsis (Ach.)
Lumbsch, Squamarina lentigera (Weber)
Poelt, and Fulgensia subbracteata,
mosses such as Pleurochaete
squarrosa (Brid.) Lindb., and Didymodon acutus (Brid.) K. Saito. (Figure 2; see Table 1 for a
species checklist). Bare soil (i.e. areas that do not contain perennial
vegetation nor well-developed BSC communities) and BSC-dominated areas cover
approximately 28% and 32% of the AES, respectively.

The research program of the Maestre labat the AES
focuses on three main topics: i) biological soil crusts and their effects on
ecosystem structure and functioning, ii) effects of climate change on BSCs and
on the ecosystem processes depending on them, and iv) plant-plant interactions.

Biological soil crusts and their effects
on ecosystem structure and functioning

Biological soil crusts –communities dominated by lichens, mosses,
bacteria and fungi that inhabit the first mm of the soil surface– are a
prevalent biotic component of dryland ecosystems worldwide that have multiple
ecosystem roles in semiarid ecosystems such as the AES (see Maestre et al. 2011
and Castillo-Monroy & Maestre 2011 for recent reviews; click here for pictures of the BSC present at the AES). A core
goal of our research at the AES is to understand how dryland ecosystems
function; therefore, if we aim to achieve this goal we need to understand what
BSCs, a key community in these ecosystems, are doing! The study of the ecology
of BSCs, and of their effects on ecosystem structure and functioning, has been
one of the core research activities carried out at the AES by the Maestre lab during the last seven years. Using natural and
manipulative field experiments, as well as laboratory experiments with
soils/BSCs from the AES, our research activities have focused on understanding
how BSCs affect the water, carbon (C) and nitrogen (N) cycles, and how BSCs
interact among them and with plant and microbial communities.

We started our N cycle studies by evaluating temporal changes in the availability of N in the six most frequent microsites found at the AES
(Castillo-Monroy et al. 2010): S. tenacissima tussocks (ST), Retama sphaerocarpa shrubs (RS), and
open areas with very low (BS), low (LC) medium (MC) and high (HC) cover of well
developed and lichen-dominated BSCs (Figure 3).
Over a two-year period (2007-2008), the temporal dynamics of available N followed changes
in soil moisture. Available NH4+-N did not differ between
microsites, while available NO3--Nwas
substantially higher in the RS than in any other microsite (Figure 4). No
significant differences in the amount of available NO3--Nwere found between ST and BS microsites,but these microsites
had more NO3--Nthan those dominated by BSCs
(LC, MC and HC). We also determined the proportion
of nitrate, ammonium and dissolved organic nitrogen (DON) was determined in all
microsites (Delgado-Baquerizo et al. 2010). DON was the dominant N form for BS
microsites, while nitrate was dominant under the canopy of Retama; these microsites contained the lowest and highest N
availability, respectively. Under BSCs, DON was the dominant N form. These results highlight the
importance of this biotic community as a modulator of the availability of N in
semi-arid ecosystems. We have been continuing
the collection these data, and right now we have over six years of N
availability data that are being analyzed (we plan to have these results ready
later in 2013).

We have also evaluatedhowperennialplantsandBSCsaffectedsmall-scalespatialpatternsofsoilinorganicN(ammoniumandnitrate)availability at the AES byusinggeostatisticalmethods (Delgado-Baquerizo et al. 2013a).Therangeofsemivariogramsforammoniumandnitrate,andthecoefficientofvariationofnitrate,werelowerinBSC-dominatedmicrosites
than in plant-dominated microsites.TheseresultssuggestthatBSCs modulate the small-scale spatial pattern
of inorganic N, producing more homogeneous conditionsforspatialdistributionofinorganicN
formsthanmicrositesprovidedbyplants.

Measurements of soil CO2 efflux
(soil respiration) at the dominant microsites of the AES (Figure 3) have been
carried out on a monthly-quaterly basis since November 2006. As part of the PhD
of Andrea Castillo-Monroy (you can download it from here) we set up an experiment to study the contribution of BSCs to the amount
of CO2 respired by the soil at the AES. The analyses of the data gathered between November 2006 and June 2010 (Castillo-Monroy et al. 2011)
revealed that soil respiration rates did not differ among BSC-dominated
microsites, but were significantly higher and lower than those found in bare
ground areas and Stipa (ST) microsites,
respectively. A model using soil temperature and soil moisture accounted for
over 85% of the temporal variation in soil respiration throughout the studied period. Using this model, we estimated a
range of 240.4-322.6 g
C·m-2·yr-1 released by soil respirationat our study area.
Vegetated (ST and Retama) and
BSC-dominated microsites accounted for 37% and 42% of this amount, respectively
(Figure 5). Our results indicate that BSC-dominated areas are the main
contributor to the total C released by soil respiration at the AES, and
therefore BSCs must be considered when estimating C budgets in drylands. These
measurements are being continued, and right now we have a ~7 year soil respiration database that will be analyzed in the upcoming
months.

Figure
5. Relative
contribution of Retama sphaerocarpa
(RS), Stipa tenacissima (ST), bare
soil (BS) and biological soil crust-dominated (BSC) microsites to the total
amount of carbon released by soil respiration throughout the study period, both
in dry (D) and wet (W) seasons. Results in 2006 represent only data from
November and December; those from 2010 are calculated using data from January
to May.

We
have also studied how BSCs impact different parts of the hydrological cycle.
Since 2006 we are continuously monitoring surface soil moisture (0-5 cm depth)
at the main microsites of the AES (Figure 3), and using this dataset Miguel
Berdugo is analyzing how BSCs control the temporal dynamics of soil moisture
(this is work in progress, so we do not have any results to show yet!). During a research visit done by David Eldridge in 2009, and with the
collaboration of our post-doc at the time Matthew A. Bowker, we used
a systems approach to examine the interactive effects of three engineers-Stipa, BSCs, and the European rabbit (Oryctolagus cuniculus)- on infiltration processes at the AES. We measured the early (sorptivity) and later
(steady–state infiltration) stages of infiltration using disk permeameters,
which allowed us to determine the relative effects of engineers on water flow
through large macropores (see Eldridge et al. 2010 for details). We detected
few effects under tension when flow was restricted to matrix pores, but under
ponding, sorptivity and steady–state infiltration adjacent to Stipa tussocks were 2–3 times higher
than in intact or rabbit–disturbed crusts (Eldridge et al. 2010). Structural Equation Modeling (SEM) showed that both Stipa and crust cover exerted
substantial and equal positive effects on infiltration under ponding, while
indirectly, rabbit disturbance negatively affected infiltration by reducing
crust cover (Figure 6). Additional SEM analyses demonstrated that 1) Stipa primarily influenced crusts by
reducing their richness, 2) rabbits exerted a small negative effect on BSC
richness and 3) lichens were negatively, and mosses positively, correlated with
a derived “infiltration” axis. The results of this experiment highlight the
importance of BSCs as a key player in the maintenance of infiltration–runoff
dynamics in Stipa grasslands, and
demonstrate the modulating role played by rabbits through their surface
disturbances.

Dew is an important source of water in drylands, particularly for BSCs, albeit its effects on the cycling
of nitrogen (N) and carbon (C) in BSC-dominated ecosystems are largely unknown.
To evaluate the effects of BSCs and dew on
N and C cycling, intact soil cores from
either bare ground or BSC-dominated micrositeswere incubated under
control and artificial dew addition treatments
(Delgado-Baquerizo et al. 2013b). A
positive increment in the amount of total available N and phenols was observed
in response to dew events under BSCs (Figure 7). We also found an increase in the
concentration of dissolved organic N, as well as in the pentoses:hexoses ratio,
under BSCs, suggesting that dew promoted an increase in the decomposition of
organic matter at this microsite. The increase in the amount of available N
commonly observed under BSCs has been traditionally associated with the
fixation of atmospheric N2 by BSC-forming cyanobacteria and
cyanolichens. Our results provide a complementary explanation for such an
increase: the stimulation of microbial activity of the microorganisms
associated with BSCs by dew inputs. These effects of dew may have important implications for nutrient cycling in
drylands worldwide, where dew events are common and BSCs cover large areas.

Not all our BSC research
has focused on the effects of these organisms soil variables and ecosystem
processes/functioning! In collaboration with Adrián Escudero, Ana Sánchez and
Arántzazu López de Luzuriaga, from the Biology & Department at the URJC, we have also evaluated how BSCs, water availability,
perennial species (presence/absence of Stipa)
and plant-plant interactions shape annual communities from the AES (Luzuriaga
et al 2012). This study revealed that water stress acted as the primary filter
determining the species pool available for the assembly of annual communities
at the AES. Stipa and BSCs acted as
secondary filters by modulating the effects of water availability. At extremely
harsh environmental conditions, Stipa
exerted a negative effect on the annual plant community, while at more benign
conditions it increased annual community richness. Biological soil crusts
exerted a contradictory effect depending on climate and on the presence of Stipa, favoring annuals in the most
adverse conditions but showing repulsion at higher water availability
conditions.

Effects of climate change on BSCs and on the ecosystem processes
depending on them

Biological soil crusts are a great study system to explore multiple
questions in community and ecosystem ecology (see Bowker et al. 2010 for a
review), and their size and characteristics make them particularly suitable to
explore climate change impacts on biotic communities and on the ecosystem
processes depending on them.

As part of the PhD of Cristina Escolar, and with the funding of a
Studentship awarded by the British Ecological Society, We started
in 2008 a manipulative experiment to evaluate responses of the BSC community
and its main constituents to a ~2.4 ºC increase in air temperature, and to a
~30% reduction of total annual rainfall, climatic conditions that mimic those
forecasted for the last half of the 21st century at the AES. We
established a factorial experimental design with three factors, each with two
levels (Figure 8): BSC cover (poorly developed BSC communities with cover <
25% vs. well developed communities with cover > 75%), warming (control vs. a
2.4ºC annual temperature increase) and rainfall exclusion (control vs. a ~30%
rainfall reduction in total annual rainfall). The working plots (1.2 m × 1.2 m)
were randomly placed either on bare ground (8.6% ± 0.8 of BSC cover; mean ± SE,
n = 40; hereafter Bare plots) or BSC-dominated (73.8% ± 1.7 of BSC cover; mean
± SE, n = 40; hereafter Crust plots) microsites. See Escolar et al. (2012) for
additional details on the experiment.

Figure 8. View of an
experimental plot with an open top chamber and a rainfall shelter in November
2008, after the full set up of the climate change experiment (left), and of
part of the area where this experiment was deployed (right). Photographs by
Cristina Escolar. Click here for additional
pictures of this experiment.

Three years after the set up of the experiment, warming promoted a
significant decrease in the richness and diversity of the whole BSC community
(Figure 9, Escolar et al. 2012). This was accompanied by important
compositional changes, as the cover of lichens suffered a substantial decrease
with warming (up to 40% on average), while that of mosses increased slightly
(from 0.3% to 7% on average). The physiological performance of the BSC
community, evaluated using chlorophyll fluorescence, increased with warming
during the first year of the experiment, but did not respond to rainfall
reduction. These results indicate that ongoing climate change will strongly
affect the diversity and composition of BSC communities, as well as their
recovery after disturbances.

The monitoring of this experiment continues, and in addition to the
monitoring of the composition, cover and physiological performance of the BSC
community, we have been measuring over the years multiple variables related to
the C (soil organic C, soil respiration, net CO2 exchange, activity
of the enzyme β-glucosidase, phenols, aromatic comounds, pentoses, and hexoses),
P (phosphate and activity of the enzyme phosphatase) and N (in situ ammonium
and nitrate availability, potential N mineralization) cycles, as well as the
microbial community (abundance of fungi, bacteria and archaea). Nowadays we are
analyzing all these data, and the article summarizing the C cycle responses to
the first four years of climate change treatments is right now under review.

As part of his PhD, Manuel Delgado-Baquerizo conducted different
experiments using soils and BSCs from the AES (Delgado-Baquerizo et al. 2013c,
2013d) to examine the role of BSCs in the resistance and
resilience of N cycle (i.e. N availability, relative dominance of N forms and
microbial biomass N) facing changes in temperature (from 5 to 30ºC) and water
soil content (from 30 to 80% of water holding capacity), as well as in the ratios
of C, N and P. Compared to bare ground areas, BSC-dominated microsites
increased the resistance and resilience in N cycle variables to changes in
temperature and in the C and N ratios, respectively. However, changes in soil
water content did not affect the N cycle in neither biocrusts nor bare grounds
areas, suggesting that processes such as mineralization in drylands have the
same activity from 30 to 80% of water holding capacity.

Figure 10. View of an experimental plot used to
simulate future climatic conditions in the Aranjuez Experimental Station during
a winter sunrise. It isn´t a nice place to work? Photograph by Beatriz Gozalo.

Plant-plant interactions

During the PhD of Santiago Soliveres (you
can download it from here) we have studied how Stipa tenacissima interacts with
different shrub species (Retama
sphaerocarpa and Lepidium subulatum)
and annuals, and how the outcome of this effect is modulated by ontogeny and
environmental stress (Figure 11).

Using the combination of spatial pattern analysis, fruit production
surveys, carbohydrate assays, sowing experiments and dendrochronological
techniques, we explored the interaction between Stipa tenacissima and Lepidium
subulatum in South- and North-facing aspects of the AES (Soliveres et al.
2010). This battery of techniques allowed us to study the effects of the nurse
plant during the whole life cycle of the protégée, and to assess the role of
spatio-temporal variability in abiotic stress as a modulator of ontogenetic
shifts in plant-plant interactions. Our results suggested a net facilitative
effect of S. tenacissima on L. subulatum, which was particularly
important during the germination of the later. This facilitative effect shifted
to competition (growth reduction) early after establishment, which was also
gradually reduced as the shrub aged (suggesting niche differentiation, Fig. 12).
The magnitude of competition was reduced under low rainfall levels in
south-facing slopes, whereas this response was observed due to other abiotic
factors in north-facing slopes.

Figure 12. Relationships between the effect size of Stipa tenacissima on the growth of Lepidium subulatum, as measured by the
RII index, and the age of Lepidium. Each
RII value is obtained by averaging growth data from 12-16 Lepidium individuals. Source: Soliveres et al. (2010).

In a series of
experiments running in parallel, we evaluated the interplay between biotic
(herbivory by the rabbit Oryctolagus
cunniculus) and abiotic (changes in water availability and frequency)
factors as drivers of the outcome of the interaction between Stipa and Retama (Soliveres et al. 2011, 2013). Stipa protected
Retama seedlings from rabbit herbivory during the wetter conditions of
spring and winter, but this effect disappeared when rabbit pressure on Retama
increased during summer drought. Stipa exerted a negative effect on
the survival of Retama seedlings during the first three years of the
experiment, which was mainly due to excessive shading. However, Stipa increased
Retama recovery after initial rabbit impact, overriding in part this
negative shade effect. The negative effects of Stipa on the photochemical efficiency of Retama juveniles
decreased with higher water availabilities, but these effects did not extent to
the survival and growth of Retama juveniles. Such effects also varied
with the intra-annual water dynamics and its experimental manipulation, overall
contradicting predictions from the stress – gradient hypothesis (i.e. increase
in facilitative interactions with increasing abiotic stress)

Overall, facilitation
research carried out at the AES highlights the crucial effect that positive
interactions at early life-stages have to determine the long-term outcome of a
given plant-plant interaction, and the existence of multiple shifts between
facilitation and competition along different life-stages of the protégée. They
also show how these ontogenetic shifts are modulated by abiotic factors, which
differ among slope aspects. The complex interactions found between herbivory,
microclimatic amelioration from Stipa, and water availability as drivers
of Retama performance illustrate the importance of considering the temporal
dynamics of both biotic and abiotic stressors to fully understand the outcome
of plant – plant interactions.

The future of research at the Aranjuez
Experimental Station

The different experiments and environmental
monitoring sensors deployed over the last seven years make the AES a very
interesting and unique research facility. As far as we can get funding to work
there, we will continue the monitoring of the different experiments we have set
up at the AES over the years. This is precisely a main goal of the work we are carrying out at the AES: to establish
there the first Long-Term Ecological Research (LTER) station devoted to monitor
soil CO2 and N fluxes in dryland areas of Europe, and to assess how
biotic communities and the ecosystem processes depending on them will respond
to ongoing climate change.

In addition to the long-term monitoring of the experiments described
above, over the last years we have set up new and ongoing experiments in
collaboration with colleagues from other institutions. With José I. Querejeta
and Lupe León, from CEBAS-CSIC, we have set up new climate change plots to evaluate the effects of increased
temperatures and reduced rainfall on the physiology and growth of Helianthemum and Lepidium. With Ana Rey and María Almagro, from MNCN-CSIC, we are studying how
UV radiation affects the decomposition of Stipa
litter (Fig. 13). Before next August we also hope to have ready a new field
experiment on litter decomposition at the AES, which will be part of the PhD of
Miguel Berdugo.

Figure 13. Partial view of the litter decomposition
experiment set up by Ana Rey at the Aranjuez Experimental Station. Photograph
by Fernando T. Maestre

The research carried out at the AES has been possible thanks to the
support of the following organizations and funding agencies: British Ecological
Society (Early Career Project Grant 231 ⁄ 607 and Studentship 231/1975),
Fundación BBVA (Intercambio
project), Comunidad de Madrid (S-0505/AMB/0335), Universidad Rey Juan Carlos
(URJC-RNT-063-2), Spanish Ministry of Science and Innovation (CGL2008- 00986-E/BOS
grant) and European Research Council (BIOCOM project). And to finish this
post a big thanks to all the people that have been working at the AES over the
years. In addition to the students and colleagues named above, we must mention
the different technicians (María D. Puche, Victoria Ochoa, Rebecca Mou, Alicia
Puche, Beatriz Gozalo, Patricia Alonso, Jorge Papadopoulos, and Marta Carpío), students
(Pablo García-Palacios, Enrique Valencia), collaborators (Antonio Gallardo, Roberto
Lázaro), post-docs (Matthew Bowker, José Luis Quero and Raúl Ochoa-Hueso) and
visitors (David Eldridge, Juan Diego Mentruyt, Juan Gaitán, Riin Tamme and different students
from Pennsylvania State University) that have been working and helping with the
different experiments set up at the AES. Without their hard work and enthusiasm
we would not have been able to conduct all the different research activities
highlighted above.

Figure 14. From up to down, and left to right: i) Andrea
measuring soil respiration, ii) a few of us trying to get the van out of the
mud, iii) Miguel and José Luis measuring net CO2 exchange at
sunrise, iv) Bea, Victoria and José Luis getting ready for a fun day of
measurements during winter, and v) Fernando explaining what we do in Aranjuez to
some visitors. Photographs by Fernando T. Maestre, Miguel Berdugo, Santiago
Soliveres and Miguel García.

Check out our twitter account and webpage for further
news and research outputs from the Aranjuez Experimental Station!

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We are looking for a motivated candidate to apply to a competitive call from the Regional Government of Madrid for funding a 4-yr post-doctoral contract to work in our lab. The requeriments of the candidates are the following:

* Strong publication record in any of the areas related to the research we do in the lab (for details about we do and recent publications check our webpage). We are particularly interested in candidates with expertise in microbial ecology, bioinformatics or remote sensing.* The candidate should hold a PhD awarded between 1 January 2013 and 20 June 2016* The candidate should have been working (under a contract or fellowship, proof required) outside Spain for at least one of the last two years.

Please note that this is not a job offer, as the candidate will apply to a competitive call (a total of 25 contracts will be awarded for all knowledge areas). All the details of this call (in Spanish) can be found here. If awarded, the person would work in the Dryland Ecology…

We are looking for a motivated candidate to apply to a competitive postdoctoral call ("Juan de la Cierva formación") from the Spanish Government that fully fund a 2-yr postdoctoral contract to work in in the Dryland Ecology and Global Change Lab. The requeriments of the candidate are the following:

* We are particularly interested in candidates with expertise in any of the following areas: microbial ecology, bioinformatics, lichen taxonomy, biocrust ecology, remote sensing or ecological modelling.* The candidate should hold a PhD awarded between 1 January 2015 and 31 December 2016.* The candidate should have a strong publication record, with articles in journals included in the first quartile of the JCR database (the call is highly competitive and succesfull candidates are those with a strong publication record).* Previous research stays and/or postdoctoral experience outside Spain (for Spanish candidates) or a Phd from a non-Spanish university (for candidates from outside Sp…